The present invention generally relates to data transmission units, and more particularly to a data transmission unit which enables the use of one line by a plurality of terminals.
In a data transmission using a switched line, it is necessary to use a data transmission unit such as a modem. On the other hand, the switched line may employ the 2-wire half duplex communication system or the 2-wire full duplex communication system. Regardless of which one of these communication systems is employed, there is an increased demand to support a plurality of terminals by a single data transmission unit.
FIG. 1 shows an example of a conventional switched line modem. In FIG. 1, a modem 3 only has one terminal connection port. For this reason, a plurality of terminals, that is, two terminals 2a and 2b in this case, cannot be connected directly to the modem 3. In this example, a switch 4 is coupled between the modem 3 and the terminals 2a and 2b. This switch 4 is used to switch the modem interface manually.
On the other hand, the transmission modes employed by the plurality of terminals may be different. In this case, each terminal must be coupled to an independent modem as shown in FIG. 2. In FIG. 2, the terminal 2a is coupled to the switch 4 via a modem 3a, while the terminal 2b is coupled to the switch via a modem 3b which employs a transmission mode different from that employed by the modem 3a.
However, according to the conventional switched line modem shown in FIG. 1, there are the following problems. First, the switch 4 must be switched manually and it is troublesome to carry out such a manual switching. Second, if the modem 3 has a plurality of transmission modes such as the 2-wire full duplex and 2-wire half duplex communication modes, it is necessary not only to switch the modem interface but also recognize the set transmission mode of the modem 3 so that the modem setting may be changed to the appropriate transmission mode if necessary.
On the other hand, there are the following problems according to the conventional switched line modem shown in FIG. 2. First, the switch 4 must be switched manually and it is troublesome to carry out such a manual switching. Second, it is necessary to provide two modems 3a and 3b. In other words, it is necessary to provide a number of modems corresponding to the number of terminals, and the construction of the switched line modem becomes complex.